Blanket wire utilizing positive temperature coefficient resistance heater

Abstract

A heating element for use in an electric blanket or the like including conductors spaced apart in a positive temperature coefficient (PTC) material which serves as a self-limiting heater. The conductors are separated by a spacer which prevents the conductors from engaging each other when the PTC material softens or melts during annealing thereof. A coating of material having a higher melting point than the PTC material is placed over the PTC material to maintain its shape during the annealing process.

What is claimed as new and desired to be secured by Letters Patent of the United States is: 
     
       1. A self-limiting electric heating element adapted for use in flexible heated products such as heating pads, electric blankets and the like comprising an elongated conductor including a flexible insulating core supporting a pair of conductor wires disposed helically around said core, an envelope of positive temperature coefficient material positioned in direct engagement with said spaced conductors providing a conducting path between said conductor wires through said material, a first coating of insulating material enclosing said positive temperature coefficient material, said coating having a melting point above the melting point of said positive temperature coefficient material to maintain the shape during the annealing operation at a temperature above the melting point of said positive temperature coefficient material and below the melting point of said coating, an insulating spacer positioned between said conductors to prevent electrical contact and maintain substantial spacing between said wires during annealing of said positive temperature coefficient material, and a second coating of insulating material surrounding said first coating. 
     
     
       2. The heating element of claim 1 wherein said conductor wires and said spacer are helically disposed on said core, the spacer being of substantially the same diameter as said wire to maintain a substantial spacing between said wires. 
     
     
       3. The heating element of claim 1 wherein said material is in the form of a flexible tape which is wrapped helically over said conductor wires and said spacer thread, said first coating being extruded over said layer of material. 
     
     
       4. The heating element of claim 1 wherein said positive temperature coefficient material is extruded around said core, said conductor wires and said spacer being wound helically around said extruded material, said first coating being extruded over said wires and said material and being a thermoplastic material.